This disclosure is related to the field of neutron well logging measurements for determining petrophysical properties of subsurface formations traversed by a wellbore. More specifically, the disclosure relates to using neutron induced gamma ray spectroscopy to determine petrophysical parameters of such formations.
Various neutron based measurements have been used to evaluate characteristics of subsurface formations from a wellbore since at least the 1950s. Neutrons can interact with subsurface formations in different ways. They can be scattered elastically, which means kinetic energy and momentum are conserved; they can be scattered inelastically, which means certain nuclei go into an excited state while kinetic energy is lost; they can also be captured by a nucleus to form a new nucleus; it is also possible that the neutron interaction causes a nuclear reaction resulting in the emission of one or more nucleons from the target nucleus. The probability of a neutron interacting with a nucleus is measured by the respective interaction cross section, which is a function of many parameters, such as incident neutron energy, outgoing neutron energy (if a neutron emerges from the interaction), scattering angle, interaction type and interactive nucleus type, among others. Thus, neutrons can enable measurement of many different formation properties due to the variety and complexity of their interactions.
One wellbore neutron measurement analysis technique known in the art is neutron induced gamma ray spectroscopy. In such techniques, gamma rays that result from inelastic collision of high energy neutrons (approximately 1 million electron volts or more) with certain nuclei in the formations. The resulting gamma rays are spectrally characterized (i.e., counted with respect to energy thereof). The spectrally characterized gamma rays are analyzed with respect to characteristic energy of gamma rays emitted by inelastic collision with known elements (called “standard spectra”). The analyzed gamma ray spectrum may be used to determine fractional amounts of each of a plurality of specific chemical elements in the formations using standard spectra. A similar analysis technique may be performed using capture gamma rays, i.e., gamma rays emitted when neutrons at lower energy level such as epithermal or thermal energy are captured by specific atomic nuclei in the formations.
There are two substantial challenges in neutron-induced gamma ray spectroscopy used in well logging. One is precision of the relative factional amounts of each element (the “yields”), the other is the accuracy. Generally speaking, the more chemical elements for which yields are to be determined from a given set of spectrally characterized neutron induced gamma rays, better accuracy in calculating yields may be obtained. For an example, if in a spectral analysis the Barium standard spectrum is excluded from the analysis while there are barite-containing fluids in the wellbore, such exclusion will create biases on other calculated elemental yields. However, the more elements to be resolved by spectral analysis of induced gamma rays, worse statistical noise will result in the determined elemental yields. That is because any elemental standard spectrum somewhat correlates with other elemental standard spectra; thus in multiple elemental analysis more standard spectra used therein may result in more correlations which will amplify the statistical noise in the raw (measured) spectra. In this manner, the accuracy and precision are a trade-off.